The increasing scarcity and the realization of ecological and safety problems associated with non-renewable energy reserves, such as coal, petroleum and uranium, have made it apparent that it is essential that increased use be made of alternate non-depletable energy resources such as photovoltaic energy. Photovoltaic use has in the past been limited to special applications in part due to the high cost of manufacturing devices capable of producing photovoltaic energy. The development of a process that continuously deposits successive layers of amorphous semiconductor alloy material on an elongated substrate to fabricate photovoltaic devices in mass production has greatly promoted the use of photovoltaic energy.
Recently, considerable efforts have been expended to develop systems and processes for preparing thin film amorphous semiconductor alloy material which can be deposited so as to form p-type and n-type semiconductor alloy layers which can encompass relatively large areas for the production therefrom of thin film photovoltaic devices. It should be noted at this point that the term "amorphous" as used herein, is defined to include alloys or material exhibiting long range disorder, although said alloys or materials may exhibit short or intermediate range order or even contain crystalline inclusions.
In the economical continuous processing, a sheet of substrate material may be continuously advanced through a succession of operatively interconnected, environmentally protected deposition chambers, wherein each chamber is dedicated to the deposition of a specific layer of semiconductor alloy material onto the sheet or onto the previously deposited layer. For example, in making a solar cell of p-i-n type configuration, the first chamber is dedicated for depositing a p-type amorphous silicon semiconductor alloy, the second chamber is dedicated for the deposition of a layer of substantially intrinsic amorphous semiconductor alloy material and the third chamber is dedicated for depositing an n-type amorphous silicon semiconductor material. Since each deposited semiconductor alloy, and especially the intrinsic semiconductor alloy must be of high purity, the environment in the deposition chambers, particularly in the intrinsic deposition chamber, is isolated from the deposition constituents within the other chambers. The diffusion of the dopant constituents from the p-type and n-type alloy deposition chambers to the intrinsic deposition chamber is halted and the contamination of the intrinsic process gases in the intrinsic deposition chamber by dopant gases is prevented.
Gas gates have been used to isolate the adjacent deposition chambers. Some known gas gates contemplate the creation of a plurality of magnetic fields adapted to urge a magnetic sheet of substrate material against the wall of the gas gate passageway opening so that the height dimension of the passageway can be reduced. The reduced height of the opening correspondingly decreases the quantity of processed gas which back diffuses from the dopant deposition chambers to the intrinsic deposition chamber without correspondingly increasing the risk that the amorphous semiconductor layers deposited on the substrate contact and be damaged by a wall of the gas gate passageway opening.
Deposition processes which maintain gas pressures of approximately 5.times.10.sup.-1 torr and above use gas gates through which a unidirectional flow of process gases from the intrinsic deposition chamber to the dopant deposition chamber is established with introduction of an inert gas which may be "swept" about the sheet of substrate material toward the dopant deposition chambers.
The sheet of substrate passing through the magnetic gas gates divides the passageway opening into a relatively wide lower slit and a relatively narrow upper slit. The velocity of the inert sweep gases and the residual process gases traveling through the wide lower slit is sufficiently great to substantially prevent the back diffusion of process gases from the dopant deposition chamber to the intrinsic chamber. However; due to the fact that the sweep gas and the deposition process gases are viscous, which viscosity becomes more pronounced at the elevated temperatures required for the glow discharge deposition of amorphous semiconductor layers onto the substrate, the drag on the sweep gases along the upper passageway wall and the unlayered surface of the substrate which define the relatively narrow upper slit results in a relatively low velocity flow therethrough. If left uncorrected, the low velocity flow is insufficient to prevent back diffusion therethrough.
Developments to correct the relatively low velocity in the narrow slit portion of the passageway introduce a plurality of grooves extending the length of the gas gate in the upper surface of the narrow slit through which high velocity inert gases sweep the dopants. The gas gate by being at least 8 inches long would create a virtually 100% probability that a process gas molecule from a deposition chamber enters one of the grooves during its back diffusion such that it is swept by the high velocity gases.
However, recent developments have made it desirable to operate deposition chambers at pressures lower than 1 millitorr. In this range of pressures, a gas gate using pressure differentials and inert gas flow, for sweeping dopant gas molecules needs a passageway opening of such narrow tolerances about the sheet of substrate that they are presently unachievable with today's best manufacturing tolerances. Furthermore, as pressures go down, the number of molecular collisions also decrease so that the length of the gas gate must be significantly lengthened to effectively stop back diffusion from one of the dopant chambers to the intrinsic deposition chamber. Significant lengthening of each gas gate between the plurality of deposition chambers would result in a significantly longer assembly line that is desirably to be avoided.
What is needed is a gas gate that can have a relatively short length and be effective in isolating dopant process gases in the dopant deposition chambers from back diffusing into the intrinsic deposition chamber for a wide range of deposition processing pressures including low pressures in the 1 millitorr range.